feat(config): allow user-defined outDir in config

Update UserConfig to accept an optional outDir and resolve it the same way vitepress resolves srcDir. The original .vitepress/dist behavior is provided as fallback if no user-provided outDir is present. For reference, see vuejs#170.

src/node/config.ts

@@ -47,6 +47,7 @@ export interface UserConfig<ThemeConfig = any> {
 
   srcDir?: string
   srcExclude?: string[]
+  outDir?: string
 
   /**
    * Enable MPA / zero-JS mode
@@ -94,6 +95,7 @@ export async function resolveConfig(
   const [userConfig, configPath] = await resolveUserConfig(root, command, mode)
   const site = await resolveSiteData(root, userConfig)
   const srcDir = path.resolve(root, userConfig.srcDir || '.')
+  const outDir = path.resolve(root, userConfig.outDir || resolve(root, 'dist'))
 
   // resolve theme path
   const userThemeDir = resolve(root, 'theme')
@@ -121,7 +123,7 @@ export async function resolveConfig(
     themeDir,
     pages,
     configPath,
-    outDir: resolve(root, 'dist'),
+    outDir,
     tempDir: path.resolve(APP_PATH, 'temp'),
     markdown: userConfig.markdown,
     alias: resolveAliases(themeDir),